The temperature and the time at which a food (i.e. meat) has been cooked is very important from the point of view of health regulations. For instance, many international animal health regulations require that importation of meat from countries in which the foot and mouth disease virus is present, be restricted to meat which has been cooked at a combination of temperature and time sufficient to inactivate the virus. Therefore, it is important to monitor the temperature history of meats during the cooking process.
This invention relates to a time-temperature integrator indicator particularly suitable for meats (or meat products) which are cooked in hermetically sealed packages and permits the verification of the thermal history during cooking (at 85.degree.-100.degree. C.). without the need of opening the package for inspection. A time-temperature integrator is a device which registers a response according to the combined effect of time and temperature. In the present invention, the time-temperature integrator is based on color changes produced by the chemical reaction between reducing sugars and amino-acids, peptides or proteins, and usually known as the Maillard's reaction.
Methods currently accepted for monitoring the cooking process of meats for export in countries where the foot and mouth disease (FMD) (or other diseases) are endemic, are of a qualitative and subjective nature. At present, most meat exporters from countries where FMD virus is endemic rely upon the so-called "pink-juice test" which is highly subjective and can not be applied to meat pieces smaller than a cube of 11/2 inch. It is relatively easy to determine the time-temperature conditions during cooking to inactivate the FMD virus in packaged meat pieces (cubes, ground, patties) or meat dishes. For a given package geometry, package size, physical-chemical characteristics of the product and heating method it is possible to find the time and temperature to inactivate the virus during cooking. However, what is more difficult is to have an objective method to verify that each single package of meat has been subjected to the time-temperature conditions required to inactivate the FMD virus. Most recently a system of temperature indicators (heat sensitive discs) which are suspended in the center of a bag of cooked beef (ground) was developed (Journal of Food Science, 47:388 (1982).
This type of temperature indicator shows color change when a pre-selected temperature is reached; this device provides no information concerning how long the system remained above the selected temperature, nor how far it rose above. This system permits food inspectors from import countries to verify that a given temperature has been reached in the "coldest" point of the package by opening it and checking the color of the indicator disc. Since this method requires opening the package it can not be applied to meat products individually packaged (i.e. one-portion cooked frozen meals). Under the system of inspection used in many meat-importers countries, upon arrival of an imported meat product, samples of the product are selected at random, ensuring that every carton of the product has the same chance of being inspected. This means that all meat packages must contain a temperature indicator device in its "coldest" point. Obviously, this is not possible for small portions destined to the retail market since the temperature indicator (disc) will end up in the consumer's plate. Thus, the principle of temperature only indicators placed in the center of a meat package works only for packages which are to be opened for reprocessing after removal of the temperature sensitive device.
Instead of using a temperature indicator which registers the maximum internal temperature, we are proposing to use a time-temperature integrator indicator which is intimately fixed on the surface of the package (i.e. laminated inside the plastic film of a pouch) and can be inspected without opening the package, provided that the outer layer be transparent.
The color changing chemical integrator involves the reaction between reducing sugars and amino-acids, peptides or proteins, and is known as the Maillard's reaction. At the temperatures corresponding to cooking (85.degree.-100.degree. C.) Maillard's reaction quickly shows a series of colors between white and dark brown; the intensity of the color (as measured by reflectance or by visual comparison with a standard color chart) is proportional to the time and temperature of cooking. Whether the cooking times of meats (or meat formulations) in boiling water (i.e. 98.degree.-00.degree. C.) inactivate the FMD virus strongly depends (among other factors) on the size of the package; it could vary between 40 minutes to 3 hours. The rate of color change in the indicator can be preselected for any given coking range and package size by modifying the reactivity of the reducing sugar and/or the amino group, the pH of the medium or the concentration of the reactants. In this way the proper range of colors may be obtained for monitoring the cooking history of small or large meat packages (i.e. small or large cooking times).